Determination of pacemaker wenckebach and adjustment of upper rate limit

ABSTRACT

A dual chamber pacemaker incorporating a system for distinguishing Wenckebach episodes from pacemaker mediated tachycardia episodes based on variations in VA intervals is disclosed which provides for adjusting the MTR upward in response to a threshold frequency of incidences of Wenckebach.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to the field of cardiac rhythmmanagement and more particularly to a dual chamber cardiac pacemakerincorporating a system for discriminating between pacemaker mediatedtachycardia (PMT) and simple upper rate limit Wenckebach behavior due tonormal sinus tachycardia. The system may also provide for adjusting theupper rate limit (URL) or maximum tracking rate (MTR) according to thefrequency of detected pacemaker Wenckebach events.

II. Related Art

In dual chamber pacemakers, circuitry is provided for both sensingatrial and ventricular depolarization events and for pacing one or bothof atrial and ventricular tissue. In a subject with normal sinus nodeactivity and interrupted conduction system, the pacemaker is able tosense an atrial depolarization (P-wave) and thereafter stimulate theventricle in accordance with an established AV delay interval. Thiseffectively mimics the heart's PR interval. The situation iscomplicated, however, by the possible occasional occurrence of aninterfering retrograde conducted P-wave, possibly the result of aventricular stimulating pulse, but one which is also sensed by theatrial sensing circuitry. Because the atrial sensing circuitry of thepacemaker cannot tell whether a sensed signal is a normal or retrogradeconducted P-wave it will initiate another ventricular stimulation event.This may quickly lead to PMT. To overcome this problem, dual chamberpacemakers are typically programmed to include a post-ventricular atrialrefractory period (PVARP) during which atrial events are sensed butignored. In this manner, if an atrial event occurs during PVARP due toretrograde conduction, an AV interval is not initiated and noventricular stimulating pulse is generated as a result of the atrialevent.

The addition of PVARP does not totally successfully resolve the problemeither, however, because in many pacemaker treated patients thecondition of the patient is such that the retrograde conduction timevaries or fluctuates depending upon physiologic feedback mechanisms.This means that a fixed, programmable PVARP may become relatively tooshort over time if retrograde conduction time increases and may nolonger serve to inhibit PMT. Conversely, if the PVARP is programmed tobe too long, this shortens the sensing window and as the pacing ratereaches the maximum atrial tracking rate or MTR set for the pacemaker,some of the desirable P-waves will fall inside the PVARP and be ignoredand this will result in an undesirable drop in the ventricular pacingrate. This is known as a two-to-one block. Thus, each time a P-wavefalls within PVARP and an AV block occurs for that cardiac cycle itresults in a missing cardiac cycle which is undesirable because itcauses short-term loss of AV synchrony and the subsequent loss ofcardiac output.

Pacemaker Wenckebach is another type of undesirable upper rate limitbehavior which negatively affects the patient. In pacemaker Wenckebach,as the atrial rate increases, the AV interval is lengthened so that theventricular pacing interval does not exceed the MTR. As the atrial rateincreases, a P-wave will eventually fall within PVARP and AV block willoccur for that cardiac cycle. The successive lengthening of the AVinterval leading to a missing cardiac cycle likewise causes short-termloss of AV synchrony and subsequent loss of cardiac output. Thedetection of pacemaker Wenckebach is important as an indication of apossible need for URL/MTR adjustment.

Pacemakers are implanted to typically operate within a particular beatrate or heart rate (HR) range including a particular URL or MTR which istypically picked on the conservative side by the physician who may havea minimum familiarity with the level of activity reached by the patient.The actual proper HR range for the patient may well extend above thatinitially programmed. This being the case, the pacer may repeatedlyreach the URL or MTR because of naturally occurring sinus rhythms andpacemaker Wenckebach may then occur. (As used herein, the termWenckebach, unless otherwise stated, refers to a pacemaker Wenckebachepidsode). Because URL or MTR may also be reached due to PMT, and PMTshould be stopped as soon as possible, because of the alternate loss ofcardiac output, there is a definite need to distinguish between thesetwo phenomena.

Pacemakers have been programmed with a function to determine whether apatient is in a pacemaker mediated tachycardia (PMT) by countingintervals that are atrially sensed and ventricularly paced at the URL.After a prescribed number of successive intervals are perceived to be atthe maximum rate, perhaps 16, the pacemaker assumes the existence of PMTand is programmed to extend the next PVARP a sufficient time to break orinterrupt the PMT. This PVARP extension is typically about 400-500 ms.An example of this approach is found in Walmsley et al (U.S. Pat. No.5,674,255) assigned to the same Assignee as the present invention. Thecontents of that patent are deemed incorporated by reference herein forany purpose. While this solution has been successful in interupting andcorrecting actual PMT, it is unable to predictably discriminate betweenURL events which are caused by PMT and which occur simply because thepatient exercised to the MTR/URL.

Thus, while the above determination and aleviation of PMT has been quitesuccessful, a need remains for adding a technique which would produce abetter and more sophisticated analysis of the nature of the cause of theupper rate limit behavior. In this manner, if one could reliablydetermine whether the rhythm is a PMT or a Wenckebach episode due tonormal sinus tachycardia, after a given number of Wenckebach eventswithin a prescribed time, the URL or MTR could be adjusted upward sothat the pacemaker range would self-adjust to be more in line with theactual activity level of the patient.

Further, in describing the related art and features of the presentinvention, it is believed that it would be helpful to define certainterminology. Accordingly, several definitions are presented.

Maximum Tracking Rate (MTR) or Upper Rate Limit (URL) is the maximumrate at which the paced ventricular rate will track sensed atrialevents. It is applicable to the atrial synchronous pacing modes, DDD,DDDR, VVDR and VDD and is programmable quantity typically residing inthe range of from about 50 to 185 pulses per minute.

AV Delay (AV) is the programmable time period from the occurrence of anatrial event, either sensed or paced, to a paced ventricular event. Itis a programmable quantity typically ranging between 10 and 300milliseconds and is active in DDD, DDI, DVI, DOO, VDD and the similarrate responsive modes.

Post Ventricular Atrial Refractory Period (PVARP) is defined as the timeperiod after a ventricular event, either paced or sensed, during whichactivity in the atrium does not inhibit an atrial stimulation pulse nortrigger a ventricular stimulating pulse. It is designed to avoid atrialsensing of retrograde activity initiated in the ventricle.

VA Interval is defined as the time period from the occurrence of aventricular event, either paced or sensed, to the occurrence of anatrial event, either sensed or paced.

Pacemaker Mediated Tachycardia (PMT). In DDD(R) and VDD(R) pacing modes,the pacemaker may detect retrograde conduction in the atrium, causingtriggered ventricular pacing rates as high as the MTR. This is referredto in the literature as pacemaker-mediated tachycardia or endless looptachycardia.

Total Atrial Refractory Period (TARP) is defined as the sum of the AVdelay and PVARP.

SUMMARY OF THE INVENTION

The present invention includes a system and method for dealing withperiodic patterns of tachycardia by more accurately determining theorigin of the upper rate limit (URL) behavior. The system discriminatesbetween PMT and normal sinus tachycardia and includes the capacity toadjust the URL/MTR upward by increments, possibly one, five or ten oreven up to thirty beats per minute in response to a predeterminedfrequency of Wenckebach events, based on as few as possibly three ormore events per month up to ten or more events per week.

If events are determined to be PMT, the system also may act to breakepisodes of PMT by increasing the PVARP interval to a predeterminedlonger time, say 400-500 ms, a value that insures that retrogradeP-waves are not tracked for at least one beat.

As an alternative in accordance with the present invention, the PVARPmay be extended by a value equal to the measured retrograde conductiontime plus some constant time, particularly between about 5 ms and 100ms, such as 50 ms. This method minimizes the PVARP extension necessaryto terminate the PMT.

Pacemakers of the class in which the invention is generally applicableinclude a pacer control algorithm designed to determine whether atachycardia event is a PMT or a Wenckebach event. According to theinvention, a VA stability check is added to a monitoring system thatcounts a predetermined number of intervals, generally 16, that have beenatrially sensed and ventricularly paced at the preset URL or MTR of thepacemaker and which are normally utilized to determine PMT. Inaccordance with the VA stability check of the invention, it has beendetermined that if the VA interval varies by more than a minimum amount,typically in the range of 5-50 ms, during a series of monitored beats atURL or MTR, then the event is more likely to have been caused by aWenckebach function than by PMT. This particularly may be the case ifsuccessive shortenings of the interval are noticed during the onset oftachycardia. The retrograde conduction time generally lengthens as thebeat rate increases with increased exercise and shortens with decreasedactivity. It has been found that if the VA interval varies by more thanabout 10-15%, or about 30 ms from beat to beat; the rate change isalmost always found to be charcteristic of a Wenckebach event. However,even change in the range from about 5 ms to 50 ms may well be due to anatural increase in the patient's intrinsic atrial rate and changesabove about 20 ms have a higher probability of being due to pacemakerWenckebach. Thus, a deviation range from about 5 ms to 50 ms has areasonably chance of being due to natural increases in the patient'sintrinsic atrial rate; while changes from about 20 ms to 50 ms have ahigh probability and at 30 ms or more, the deviation is almost certainto be due to natural increases in atrial rate. this value has beenselected as a non-limiting example in the detailed description below.

One successful detection system uses 32 ms as the threshold beat-to-beatVA change. This value represents about 10-15% of a typical VA interval.

If a number of naturally induced or Wenckebach events occur within agiven time period, say 10 per week, or even 3 per month, this may wellbe indicative that a condition exists in which the URL/MTR is set toolow and should be incremented by one, five or 10 or more beats perminute to better represent the actual required pacing rate range of theheart of the patient. The control program of the pacer according to thepresent invention may also contain the ability to adjust the URL/MTRupward from one to about 30 RPM automatically in response to thesituation.

OBJECTS

Accordingly, it is a primary object of the present invention to providea technique that more accurately distinguishes between upper rate limitbehavior precipitated by a PMT and a pacemaker Wenckebach event.

It is a further object of the present invention to provide a pacemakerWenckebach counter which indicates a cumulative number of Wenckebachevents in a given time which may be used to indicate that the pacemakeris programmed at too low a URL or MTR.

A still further object of the present invention is to utilize such aWenckebach counter as an input to a system that operates toautomatically alter the pacing limit to increase the URL/MTR by acertain number of beats per minute.

These and other objects, as well as these and other features andadvantages of the present invention, will become readily apparent tothose skilled in the art from a review of the following detaileddescription of the illustrated embodiment in conjunction with theaccompanying drawings in which like numerals in the several views referto corresponding parts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a general block diagram of a dual chamber pacemaker coupled toa heart by a pacemaker lead;

FIG. 2 is a block diagram of the microcontroller in the pacemaker ofFIG. 1; and

FIG. 3 is a schematic for diagram of a portion of the pacer control foratrial sensing and ventricular pacing showing one implementation of theinvention.

DETAILED DESCRIPTION

FIG. 1 depicts the operative components of an implantable dual chamberpacemaker of a class typically used for bradycardia enclosed by thebroken line box 10. The pacer system includes an actual sense amplifier12 and associated atrial pulse generator 14 which are connected by wire16 contained in a pacing lead 18 to a sensing and pacing electrode at 20located in the right atrium of heart represented by 22. The system alsoincludes a ventricular sense amplifier 24 and ventricular pulsegenerator 26 that are connected by a conductor 28 also contained in thepacing lead 18 to an associated sensing and pacing electrode 30 disposedin the right ventricle of the heart 22. It will further be recognizedthat while the foregoing description pertains to a unipolar lead system,either or both leads can be bipolar, hence a second electrode 32 isillustrated on the ventricular lead 15.

The outputs from the atrial sense amplifier 12 and the ventricular senseamplifier 24 are applied as inputs to a microprocessor-basedmicrocontroller 34 which functions to control the time of application ofatrial stimulating pulses (A_(p)) and ventricular stimulating pulses(V_(p)) to the heart in a coordinated fashion determined by the softwareexecuted by the microprocessor portion of the microcontroller 34.

The microcontroller 34 is depicted in greater detail in the blockdiagram of FIG. 2 and includes a programmable microprocessor 40 formedas an integrated circuit that can be encapsulated along with a batterypower supply 41 within a hermetically sealed can as is well known in theart. The microprocessor 40 includes a clock oscillator whose frequencyis controlled by a crystal 43 as well as the usual compliment of programcounter, instruction decode logic, register stacks and an ALU, all ofthese components being well-known and conventionally found inmicroprocessors. A variety of such microprocessors are available for usein implementations of the present invention.

The microprocessor 40 has an associated semiconductor ROM memory 42, aread/write or random access memory (RAM) 44, and a input/outputinterface 46 which are coupled to the microprocessor 40 via an addressbus 48, a data bus 50 and a control bus 52. The ROM memory 42 typicallycontains a program of instructions while the RAM memory 44 will storeprogrammable operands which may be telemetered into the implantedpacemaker 10 from an external programmer/monitor module (not shown) butwhich also is conventional in the art. In particular, the A_(s) andV_(s) inputs from the electrodes on the sensing/pacing leads 15 and 18are applied to the microprocessor 40 via the I/O interface 46 and thatinterface is also used to couple the control signals A_(p) and V_(p) tothe atrial pulse generator 14 (if used) and the ventricular pulsegenerator 26 at appropriate times as dictated by the program executed bythe microprocessor 40.

FIG. 3 depicts a software flow diagram of one possible embodiment of aprocess or algorithm that can be implemented in the software executed bythe microprocessor 40 of FIG. 2 in implementing a natural Wenckebachdetection system with means to increment the MTR usable in either arate-adaptive or non-rate-adaptive pacemaker. The process or algorithmbegins at block 60 with the initialization or programming an initialvalue for MTR values for PVARP and certain constants M, N, P and R.

As will become apparent as the description of the invention continues, Nis a natural Wenckebach count that is incremented by one for eachnatural Wenckebach episode. Episodes are accumulated for a predeterminedvalue of M or until N=5. M, then, is the number of time units, typicallymeasured in days, nominally a month or 30 days. P is a number of beatsat MTR (e.g., 16 beats) and is also used in the determination anddifferentiation of Wenckebaching from PMT. MTR at block 60 is an initialprogrammed value of MTR selected by the user. R is a BPM increment to beadded to MTR (e.g., 10 BPS).

Following the initialization steps, MTR is set equal to the programmedvalue of MTR plus the factor R at block 62. As test is then made atblock 64 to determine whether M days have gone by since a precedingincrement of MTR and, if not, control exits to decision block 66. Bymeasuring the V—V interval between successive ventricular stimulatingpulses, the ventricular pacing rate can be determined. The test at block66 determines whether the ventricular pacing rate has risen to MTR and,if not, control returns, via path 68, whereby step 62 is again repeated.

When the test at decision block 66 reveals that the pacing rate hasbecome equal to MTR, further tests are made at blocks 70, 72 and 74 with72 being a time measurement to determine whether a predetermined numberof successive ventricular beats, P, have taken place where theventricular pacing rate has remained at MTR and whether the VA variesmore than 32 ms between any beats. The value P is an arbitrary number,but a period of 16 beats has been determined to provide a sufficientsample. If the predetermined number of beats at MTR does not occur,control again returns via path 76 to the input of block 62.

When it is determined that the pacemaker is pacing the ventricle at MTRfor the predetermined number of beats, it may be indicative that a PMTor natural Wenckebach is in progress and the algorithm provides fordiscriminating at 74. If the VA period varies less than 32 ms, this isindicative that a PMT is in progress and by increasing the PVARP to,say, 500 ms, or to a value based on the measured retrograde conductiontime plus a constant time of, say, 50 ms at block 78, it is highlylikely that the PMT will be broken since PVARP approaches the AAinterval, and any retrograde conducted ventricular stimulating pulseswould have taken place during the extended PVARP.

On the other hand, in accordance with the invention, if the VA perioddoes vary by about 32 ms or more, the episode is determined to be apacemaker Wenckebach situation at block 80 and a signal on lines 82 and86 increments N at 86. When N=4 at 84 within a time period less than Mdays (possibly 30 days) at 64 this produces a control signal to be sentout from block along 90, 86 and 92 to cause an increment in MTR at 62 byR BPM and to reset M=0 and N=0 at 64 and 84, respectively. In thismanner, the valve of MTR is adjusted upward based on 4 Wenckebach eventsoccurring within M days. Of course, all the selected values, N, M, R,etc., are not meant to be limiting and are somewhat arbitrary exampleswhich can be varied as indicated to better serve any particularsituation.

This invention has been described herein in considerable detail in orderto comply with the Patent Statutes and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct and use such specialized components as are required. However,it is to be understood that the invention can be carried out byspecifically different equipment and devices, and that variousmodifications, both as to the equipment details and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

What is claimed is:
 1. A dual chamber cardiac pacemaker having an upperrate limit (URL) comprising: (a) sensor for sensing atrialdepolarization events; (b) sensor for sensing ventricular depolarizationevents; (c) output device for stimulating a ventricular chamber; (d)first timer coupled to said sensor for sensing atrial depolarizationevents for determining a VA interval and establishing an AV intervalbetween an occurrence of an atrial depolarization event and the timefollowing such occurrence at which the output device for stimulating aventricular chamber stimulates the ventricular chamber; (e) second timercoupled to said sensor for sensing ventricular depolarization events forsensing the VA interval; (f) system for detecting the occurrence ofpacemaker mediated tachycardia episodes from a predetermined interval atURL; and (g) distinguishing device for determining variations in the VAinterval determined by said timers and for distinguishing Wenckebachepisodes from pacemaker mediated tachycardia episodes based on saidvariations in VA intervals.
 2. A dual chamber cardiac pacemaker as inclaim 1 wherein the device for distinguishing Wenckebach episodes frompacemaker mediated tachycardia episodes comprises a device for comparingVA interval times for successive beats.
 3. A dual chamber cardiacpacemaker as in claim 2 wherein the distinguishing device determines theexistence of a Wenckebach episode when a beat to beat VA intervalvariation is between 5 ms and 50 ms.
 4. A dual chamber cardiac pacemakeras in claim 2 wherein the distinguishing device determines the existenceof a Wenckebach episode when a beat to beat VA interval variation ≧30ms.
 5. A dual chamber cardiac pacemaker as in claim 1 including a devicefor counting Wenckebach episodes occurring within a predetermined timeinterval and further incrementing the pacemaker upper rate limit basedon a predetermined Wenckebach episode frequency.
 6. A dual chambercardiac pacemaker as in claim 5 wherein the predetermined Wenckebachepisode frequency is ≧3 per month.
 7. A dual chamber cardiac pacemakeras in claim 5 wherein said pacemaker upper rate limit is incremented byan amount in the range of 1-30 beats per minute.
 8. A dual chambercardiac pacemaker as in claim 1 including a device responsive to thedistinguishing device for breaking the PMT episode, comprising a devicefor increasing a PVARP time interval for at least one beat to apredetermined value sufficient to insure that retrograde P-waves are nottracked by the device for stimulating a ventricular chamber.
 9. A dualchamber cardiac pacemaker as in claim 8 wherein the predetermined valueto which the PVARP time interval is increased for one beat is in therange of from 350 ms to 500 ms.
 10. A dual chamber cardiac pacemaker asin claim 8 wherein the predetermined value to which the PVARP timeinterval is increased is a value equal to a measured time (VA) plus aconstant time in the range of 5-100 ms.
 11. A dual chamber programmablecardiac pacemaker having an upper rate limit comprising: (a) means forsensing atrial depolarization events; (b) means for sensing ventriculardepolarization events; (c) means for stimulating a ventricular chamber;(d) first timing means coupled to the means for sensing atrialdepolarization events for determining a VA interval and establishing anAV interval between the occurrence of an atrial depolarization event andthe time following such occurrence at which the means for stimulating aventricular chamber stimulates the ventricular chamber; (e) secondtiming means coupled to the means for sensing ventricular depolarizationevents for sensing the VA interval and establishing a PVARP timeinterval during which the first timing means is inhibited fromestablishing an AV interval; (f) means for detecting the occurrence ofpacemaker mediated tachycardia episodes from a predetermined interval atURL; and (g) distinguishing means for determining variations in the VAinterval determined by both said timer means and for distinguishingWenckebach episodes from pacemaker mediated tachycardia episodes basedon said variations in VA interval.
 12. A dual chamber cardiac pacemakeras in claim 11 wherein the distinguishing means for distinguishingWenckebach episodes from pacemaker mediated tachycardia episodescomprises a device for comparing VA interval times for successive beats.13. A dual chamber cardiac pacemaker as in claim 12 wherein thedistinguishing means determines the existence of a Wenckebach episodewhen a beat to beat VA interval variation is in the range between about5 ms and 50 ms.
 14. A dual chamber cardiac pacemaker as in claim 12wherein the distinguishing means determines the existence of aWenckebach episode when a beat to beat VA interval variation ≧30 ms. 15.A dual chamber cardiac pacemaker as in claim 11 including a device forcounting Wenckebach episodes occurring within a predetermined timeinterval and further incrementing the pacemaker upper rate limit basedon a predetermined Wenckebach episode frequency.
 16. A dual chambercardiac pacemaker as in claim 15 wherein the predetermined Wenckebachepisode frequency is ≧3 per month.
 17. A dual chamber cardiac pacemakeras in claim 15 wherein said pacemaker upper rate limit is incremented byan amount in the range of 1-30 beats per minute.
 18. A dual chambercardiac pacemaker as in claim 11 including means responsive to thedetecting means for breaking the PMT episode comprising means forincreasing a PVARP time interval for at least one beat to apredetermined value sufficient to insure that retrograde P-waves are nottracked by the means for stimulating a ventricular chamber.
 19. A dualchamber cardiac pacemaker as in claim 18 wherein the predetermined valueto which the PVARP time interval is increased for one beat is in therange of from 350 ms to 500 ms.
 20. A dual chamber cardiac pacemaker asin claim 18 wherein the predetermined value to which the PVARP timeinterval is increased is a value equal to a measured time (VA) plus aconstant time in the range of 5-100 ms.